Dehumidification can be accomplished by mechanically lowering the dew-point of air, using a refrigeration based system, to a predetermined temperature and humidity level that removes a desired amount of moisture or by using outdoor air that is at the predetermined temperature and humidity level or lower.
In many geographic locations, dehumidification using only outdoor air is not practical because the outdoor dew point exceeds the indoor dew point too frequently. Under these conditions indoor humidity is not controlled, causing discomfort and the growth of mold and mildew. Consequently, most systems use refrigeration based dehumidification to maintain indoor humidity for some portion of the year.